Some quilters take great care in their choice of thread, using different weights for piecing, applique, and quilting. They go to great lengths to match colors precisely. Is it all cotton or polyester or silk? What about wool for applique? Pearl cotton or 50 weight? And money is no object as long as the look is just right.
Then there are those of us who use whatever we have on hand. We consider that if it was good enough for our grandmothers, who didn't have the vast array of choices we have today, it's good enough for us.